Birefringent interference polarizer

ABSTRACT

A birefringent interference polarizer which may be fabricated from readily available materials using established coextrusion techniques is provided. The polarizer has a level of light absorption near zero and can be fabricated to polarize and reflect light of specific wavelengths while transmitting light of other wavelengths. The polarizer includes multiple alternating oriented layers of at least first and second polymeric materials having respective nonzero stress optical coefficients which are sufficiently different to produce a refractive index mismatch between the first and second polymeric materials in a first plane which is different from the refractive index mismatch between the first and second polymeric materials in a second plane normal to the first plane. The refractive index mismatch in the first plane is preferably at least 0.03.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation in part of U.S. application Ser. No463,645 filed Jan. 11, 1990, now U.S. Pat. No. 5,122,906, and is also acontinuation in part of U.S. application Ser. No .466,168, filed Jan.17, 1990, now U.S. Pat. No. 5,122,905, both of which applications arethemselves continuations in part of U.S. application Ser. No. 368,695,filed Jun. 20, 1989, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to a multilayer birefringent interferencepolarizer, and more particularly to a multilayer coextruded polymericdevice which can be designed to polarize selected wavelengths of lightby constructive optical interference.

Birefringent polarizers are generally known in the art and have beenused in the past to polarize and filter selected wavelengths of light.For example, birefringent polarizers may be used to reject (reflect)specific polarized narrow wavelength ranges while transmitting theremainder of the incident light, to reduce glare from other lightsources, and to act as beam splitters.

Many naturally occurring crystalline compounds act as birefringentpolarizers. For example, calcite (calcium carbonate) crystals have wellknown birefringent properties. However, single crystals are expensivematerials and cannot be readily formed into the desired shapes orconfigurations which are required for particular applications. Others inthe art, such as Makas, U.S. Pat. No. 3,438,691, have fabricatedbirefringent polarizers from plate-like or sheet-like birefringentpolymers such as polyethylene terephthalate incorporated into anisotropic matrix polymer.

In many instances, polymers can be oriented by uniaxial stretching toorient the polymer on a molecular level such as taught by Rogers et al,U.S. Pat. No. 4,525,413. Multilayer optical devices comprisingalternating layers of highly birefringent polymers and isotropicpolymers having large refractive index mismatches have been proposed byRogers et al. However, the Rogers et al device requires the use ofspecific highly birefringent polymers having certain mathematicalrelationships between their molecular configurations and electrondensity distributions.

Accordingly, there remains a need in the art for birefringentinterference polarizers which can be readily produced using existingtechniques and readily available materials. Further, there still existsa need in the art for birefringent interference polarizers which absorblittle light. Further, the need exists in the art for birefringentpolarizers which can be fabricated to polarize light of specificwavelengths as desired.

SUMMARY OF THE INVENTION

The present invention meets that need by providing a birefringentinterference polarizer in the form of a multi-layered sheet or filmwhich may be fabricated from readily available materials usingestablished coextrusion techniques. The polarizer of the presentinvention has a level of light absorption near zero and can befabricated to polarize and reflect light of specific wavelengths whiletransmitting light of other wavelengths. The polarizer will alsopolarize the transmitted light at those wavelengths, while the remainderof the transmitted light remains unpolarized.

Reference to polarizers, polarized light, and polarization as usedherein refers to a condition of light in which the transverse vibrationof the rays assume different forms in different planes. Polarization, asused herein, includes the nonequal reflection of light in orthogonalplanes and encompasses elliptical and circular polarization of light aswell as plane polarization. By "light" we mean not only light in thevisible spectrum, but also ultraviolet and infrared light. When theplane of orientation of the polymeric materials is discussed herein, weare referring to the directions of orientation of the polymericmaterials due to uniaxial or biaxial stretching of the materials in thex and/or y direction to define the polarizing effect of the materials.In other contexts, reference to the plane that light enters or impingesupon the layers of polymeric materials is a plane normal to the majorsurfaces of the layers (i.e., the z direction), unless otherwiseindicated.

In accordance with one aspect of the present invention, a birefringentinterference polarizer is provided comprising multiple alternatingoriented layers of at least first and second polymeric materials havingrespective nonzero stress optical coefficients which are sufficientlydifferent to produce a refractive index mismatch between the first andsecond polymeric materials in a first plane which is different from therefractive index mismatch between the first and second polymericmaterials in a second plane normal to the first plane.

The birefringent polarizer of the present invention may also comprisethree or more alternating layers of diverse polymeric materials. Forexample, a three layer pattern of repeating units ABCBA may be used,where the B unit is a copolymer or miscible blend of the A and C repeatunits. In some instances, the B layer may not only contribute to thelight polarization properties of the invention, but also act as anadhesive layer to bond the A and C layers together.

Also, the third polymer layer may be found as a surface or skin layer onone or both major exterior surfaces for an ABABAB repeating body or asan interior layer. The skin layer may be sacrificial, or may bepermanent and serve as scratch resistant or weatherable protectivelayer. Further, such skin layers may be post applied to the polarizerafter coextrusion. For example, a skin layer may be applied as a sprayedon coating which would act to level the surface of the polarizer toimprove optical properties and impart scratch resistance, chemicalresistance and/or weatherability. The skin layer may also be laminatedto the multilayered polarizer. Lamination is desirable for thosepolymers which are not readily coextrudable.

In one embodiment of the invention, the first and second polymericmaterials have substantially equal refractive indices when unoriented.The refractive index mismatch develops in the plane of orientation whenthe materials are stretched. In another embodiment, the first and secondpolymeric materials have differing refractive indices when unoriented.Orienting the polymers by stretching causes the mismatch betweenrespective refractive indices in one of the planes to decrease, whilethe mismatch in the other plane is maintained or increased. Thepolarizer may be uniaxially or biaxially oriented.

In a preferred form of the invention, the first polymeric material has apositive stress optical coefficient, while the second polymeric materialhas a negative stress optical coefficient. Preferably, the refractiveindex mismatch in the first plane is at least 0.03, and most preferably0.05 or greater.

Preferably, the optical thickness of each polymeric layer is from about0.09 micrometers to about 0.70 micrometers. Optical thickness, nd, isdefined as the product of the physical thickness of the layer (d) andits refractive index (n). In a preferred form of the invention, thelayers increase in thickness monotonically through the thickness of thefilm to produce a layer thickness gradient which reflects and polarizesa broad range of wavelengths of light.

The two polymeric materials can be any of a number of different polymerswhich possess nonzero stress optical coefficients which provide thenecessary refractive index mismatch when the materials are oriented. Bynonzero stress optical coefficient, it is meant that the refractiveindex of the polymer changes in either a positive or negative directionwhen the polymer is oriented. Isotropic materials possessing zero stressoptical coefficients lack birefringence.

For example, the first polymeric material may be a polycarbonate, suchas a bisphenol A based polycarbonate, or a polyethylene terephthalate,both of which possess positive stress optical coefficients. The secondpolymeric material may be a polystyrene which has a negative stressoptical coefficient. Either generally amorphous atactic polystyrenes ormore crystalline syndiotactic polystyrenes are suitable. Other suitablepolymers for the second polymeric material include copolymers of styreneand acrylonitrile, copolymers of styrene and methyl methacrylate, andpolyethylene naphthalate, all of which possess negative stress opticalcoefficients.

The polarizer of the present invention reflects and polarizes a portionof the light incident on its surface while transmitting the remainder ofthe incident light. During fabrication, it may be designed to transmitonly a narrow range of wavelengths while reflecting a broad range, orvice versa. The polarizer of the present invention may also be designedto reflect and polarize substantially all light incident in one plane ofthe device while transmitting substantially all light incident in aplane normal thereto.

In some embodiments of the invention it may be desirable to incorporatecoloring agents such as dyes or pigments into one or more of theindividual layers of the birefringent polarizer. This can be done to oneor both of the outer or skin layers of the body, or alternatively, thecoloring agent may be incorporated into one or more interior layers inthe polarizer. The use of pigments or dyes permits the selectiveabsorption of certain wavelengths of light by the polarizer. While anunpigmented or undyed multilayer film will reflect specific polarizedwavelengths and transmit the remainder of incident light, pigments anddyes can be used to further control the bandwidth of reflected polarizedlight and the wavelength range of transmitted light. For example, alltransmitted light may be absorbed by coextruding a black layer on theback side of the birefringent polarizer. Furthermore, dyes may be usedto narrow the wavelength band of reflected polarized light andtransmitted light by absorbing selected wavelengths.

The polymers chosen will determine the refractive index mismatch,respective stress optical coefficients, and glass transitiontemperatures. The number of layers, degree of orientation, layerthicknesses, and use of pigments or dyes may all be adjusted(controlled) to provide a polarizer having the desired characteristicsfor a particular end use. This contrasts to prior art devices which werelimited both in design and polarization characteristics.

In another embodiment of the invention, a tunable birefringentinterference polarizer is provided and comprises multiple alternatinglayers of first and second elastomeric materials having respectivenonzero stress optical coefficients which are sufficiently different toproduce a refractive index mismatch between the first and secondelastomeric materials in a first plane which is different from therefractive index mismatch between the first and second elastomericmaterials in a second plane normal to the first plane. Because theindividual layers forming the polarizer are elastomers, the polarizervariably polarizes wavelengths of light dependent upon the degree ofelongation of the elastomers. Additionally, because the layers areelastomers, the polarizer is tunable and reversible as the device isreturned to a relaxed state.

The present invention also provides a method of making a birefringentinterference polarizer comprising the steps of coextruding at leastfirst and second polymeric materials having respective nonzero stressoptical coefficients in multiple layers. The layers may be stretched toorient the polymeric materials and produce a refractive index mismatchin a first plane which is different from the refractive index mismatchbetween the first and second polymeric materials in a second planenormal to the first plane. While many polymer combinations can bestretched at temperatures above the glass transition temperature butbelow the melting temperature of the polymers, some polymer combinationscan be "cold drawn," where one or more of the polymers can be stretchedat a temperature below its glass transition temperature.

In one embodiment of the invention, the first and second polymericmaterials have substantially equal refractive indices when unoriented,with a refractive index mismatch in one plane developing uponorientation. In another embodiment, when oriented, the first and secondpolymeric materials have substantially equal refractive indices in oneof the first and second planes, but there is a refractive index mismatchin the other plane. The orientation of the polymeric materials may beeither uniaxial or biaxial.

Preferably, the refractive index mismatch in the first plane is at leastabout 0.03, and most preferably at least 0.05 or greater, with theoptical thickness of each layer being from about 0.09 micrometers toabout 0.70 micrometers. In one embodiment, the layers increase inthickness monotonically through the thickness of the film to provide apolarizer which reflects a broad range of wavelengths. In a preferredform of the invention, the first polymeric material has a positivestress optical coefficient, and the second polymeric material has anegative stress optical coefficient.

Accordingly, it is an object of the present invention to provide abirefringent interference polarizer, and method of making, which may befabricated from readily available materials, using establishedcoextrusion techniques, to include having a level of light absorptionnear zero and be fabricated to reflect and polarize light of specificwavelengths while transmitting light of other wavelengths. This, andother objects and advantages of the present invention will becomeapparent from the following detailed description, the accompanyingdrawings, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of reflectance versus wavelength of light for amultilayer optical interference polarizer made in accordance with thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides improved optical interference polarizersin the form of multilayer films with a number of desirable propertiesincluding the ability to tailor the device to polarize selectedwavelengths of light. The basic optical principles involved in thepresent invention are those relating to the reflection of light by thinfilm layers having differing refractive indices. These principlesdemonstrate the dependency of the effect on both individual layerthickness as well as refractive index of the material. See, Radford etal, "Reflectivity of Iridescent Coextruded Multilayered Plastic Films",13 Polymer Engineering and Science 216 (1973).

A thin film is described in the literature as one whose thickness, d, isless than about 0.5 micrometers or whose optical thickness, nd (where nis the refractive index of the material) is less than about 0.7micrometers. Vasicek, Optics of Thin Films (1960) at pages 100 and 139.

Interference films which rely on the constructive optical interferenceof light to produce intense reflected light in the visible, ultraviolet,or infrared portions of the electromagnetic spectrum have been describedin the prior art. See, for example, Alfrey, Jr. et al, U.S. Pat. No.3,711,176. Such interference films act according to the equation:

    λ.sub.m -(2/m)(N.sub.1 D.sub.1 +N.sub.2 D.sub.2)

where λ_(m) is the reflected wavelength in nanometers, N₁ and N₂ are therefractive indices of the alternating polymers, D₁ and D₂ are thethickness of the respective layers of polymers in nanometers, and m isthe order of reflection (m=1, 2, 3, 4, 5). This is the equation forlight incident normal to the surface of the film. For other angles ofincidence, the equation will be modified to take into account the angle,as is known in the art. The polarizer of the present invention isoperable for all angles of incident light. Each solution of the equationdetermines a wavelength at which an intense reflection, relative tosurrounding regions, is expected. The intensity of the reflection is afunction of the "f-ratio" where, ##EQU1##

By proper selection of the f-ratio, one can exercise some degree ofcontrol over the intensity of reflection of the various higher orderreflections. For example, first order visible reflections of violet(about 0.38μ wavelength) to red (about 0.68μ wavelength) can be obtainedwith layer optical thicknesses between about 0.075-0.25 micrometers.

However, light reflected from prior art thin layer interference films isnot polarized. The light reflected from the alternating polymeric layersof the present invention is polarized principally due to thebirefringent nature of the film. Thus, in its preferred form, thebirefringent interference polarizer of the present invention comprisesmultiple alternating oriented layers of at least first and secondpolymeric materials having respective nonzero stress opticalcoefficients which are sufficiently different to produce a refractiveindex mismatch between the first and second polymeric materials in afirst plane which is different from the refractive index mismatchbetween the first and second polymeric materials in a second planenormal to the first plane. This refractive index mismatch is preferablyat least about 0.03, and most preferably at least 0.05 or greater. Thisconstruction results in a polarizer having optical interference in afirst plane, such as the plane of orientation, and near zero opticalinterference in a second plane normal thereto.

Preferably, the optical thickness of each polymeric layer is in therange of from about 0.09 to about 0.70 micrometers. Polymers suitablefor use in the practice of the present invention include generallytransparent thermoplastic polymers having stress optical coefficientswhich provide the necessary refractive index mismatch in at least oneplane when the polymers are oriented. Additionally, it is desirable froma processing standpoint that the polymers be compatible for coextrusion.

One example of a suitable polymer pair is polycarbonate and polystyrene.Syndiotactic polystyrene is believed to be especially suitable.Polycarbonate has a positive stress optical coefficient, whilepolystyrene has a negative stress optical coefficient. Both haverefractive indices (unoriented) of approximately 1.6. Other generallytransparent thermoplastic polymers which are suitable for use in thepresent invention include elastomers such as those described incommonly-assigned application Ser. No. 339,267, filed Apr. 17, 1989, andentitled "Elastomeric Optical Interference Films", now U.S. Pat. No.4,937,134, issued Jun. 26, 1990, the disclosure of which is herebyincorporated by reference.

Additionally, other polymers and copolymers such as polyethylene 2,6naphthalate, a copolymer based on 1,4-cyclohexanedimethyleneterephthalate (PCTG), and copolymers of gluterimide and methylmethacrylate (KAMAX resins, available from Rohm and Haas), are useful inthe practice of the present invention. Further, miscible blends ofpolymers may be used to adjust the refractive index, stress opticalcoefficient, and glass transition temperature of the layers used in thepolarizer. Other exemplary thermoplastic resins, along withrepresentative refractive indices, which may find use in the practice ofthe present invention include, but are not limited to: perfluoroalkoxyresins (refractive index=1.35), polytetrafluoroethylene (1.35),fluorinated ethylene-propylene copolymers (1.34), silicone resins(1.41), polyvinylidene fluoride (1.42), polychlorotrifluoroethylene(1.42), epoxy resins (1.45), poly(butyl acrylate) (1.46),poly(4-methylpentene-1) (1.46), poly(vinyl acetate) (1.47), ethylcellulose (1.47), polyformaldehyde (1.48), polyisobutyl methacrylate(1.48), polymethyl acrylate (1.48), polypropyl methacrylate (1.48),polyethyl methacrylate (1.48), polyether block amide (1.49), polymethylmethacrylate (1.49), cellulose acetate (1.49), cellulose propionate(1.49), cellulose acetate butyrate (1.49), cellulose nitrate (1.49),polyvinyl butyral (1.49), polypropylene (1.49), polybutylene (1.50),ionomeric resins such as Surlyn (trademark) (1.51), low densitypolyethylene (1.51), polyacrylonitrile (1.51), polyisobutylene (1.51),thermoplastic polyesters such as Ecdel (trademark) (1.52), naturalrubber (1.52), perbunan (1.52), polybutadiene (1.52), nylon (1.53),polyacrylic imides (1.53), poly(vinyl chloro acetate) (1.54), polyvinylchloride (1.54), high density polyethylene (1.54), copolymers of methylmethacrylate and styrene-(1.54), transparentacrylonitrile-butadiene-styrene terpolymer (1.54), allyl diglycol resin(1.55), blends of polyvinylidene chloride and polyvinyl chloride such asSaran resins (trademark) (1.55), polyalpha-methyl styrene (1.56),styrene-butadiene latexes such as Dow 512-K (trademark) (1.56),polyurethane (1.56), neoprene (1.56), copolymers of styrene andacrylonitrile such as Tyril resin (trademark) (1.57), copolymers ofstyrene and butadiene (1.57), other thermoplastic polyesters such aspolyethylene terephthalate and polyethylene terephthalate glycol (1.60),polyimide (1.61), polyvinylidene chloride (1.61), polydichlorostyrene(1.62), polysulfone (1.63), polyether sulfone (1.65), and polyetherimide(1.66).

Copolymers and miscible blends of the above polymers may also find usein the practice of the present invention. Such copolymers and blends maybe used to provide an extremely wide variety of different refractiveindices which may be matched to provide optimum polarizing effects.Additionally, the use of copolymers and miscible blends of polymers maybe used to enhance the processability of the alternating layers duringcoextrusion and orientation. Further, the use of copolymers and miscibleblends permits the adjustment of the stress optical coefficients andglass transition temperatures of the polymers.

Multilayer birefringent interference polarizing films in accordance withthe present invention are most advantageously prepared by employing amultilayered coextrusion device as described in U.S. Pat. Nos. 3,773,882and 3,884,606 the disclosures of which are incorporated herein byreference. Such a device provides a method for preparing multilayered,simultaneously extruded thermoplastic materials, each of which are of asubstantially uniform layer thickness. Preferably, a series of layermultiplying means as are described in U.S. Pat. No. 3,759,647 thedisclosure of which is incorporated herein by reference may be employed.

The feedblock of the coextrusion device receives streams of the diversethermoplastic polymeric materials from a source such as a heatplastifying extruder. The streams of resinous materials are passed to amechanical manipulating section within the feedblock. This sectionserves to rearrange the original streams into a multilayered streamhaving the number of layers desired in the final body. Optionally, thismultilayered stream may be subsequently passed through a series of layermultiplying means in order to further increase the number of layers inthe final body.

The multilayered stream is then passed into an extrusion die which is soconstructed and arranged that streamlined flow is maintained therein.Such an extrusion device is described in U.S. Pat. No. 3,557,265, thedisclosure of which is incorporated by reference herein. The resultantproduct is extruded to form a multilayered body in which each layer isgenerally parallel to the major surface of adjacent layers.

The configuration of the extrusion die can vary and can be such as toreduce the thickness and dimensions of each of the layers. The precisedegree of reduction in thickness of the layers delivered from themechanical orienting section, the configuration of the die, and theamount of mechanical working of the body after extrusion are all factorswhich affect the thickness of the individual layers in the final body.

After coextrusion, and layer multiplication, the resultant multilayerfilm is stretched, either uniaxially or biaxially, at a temperatureabove the respective glass transition temperatures of the polymers, butbelow their respective melting temperatures. Alternatively, themultilayer film may be cold drawn and stretched below the glasstransition temperature of at least one of the polymers in the film. Thiscauses the polymers to orient and produces a refractive index mismatchin at least one plane of the polarizer due to the differences in stressoptical coefficients and/or refractive indices between the polymers.

Polarization of selected wavelengths of light is achieved by means ofconstructive optical interference due to the refractive index mismatchin at least one plane of the polarizer. The polarizer can be constructedso that different wavelengths may be polarized as desired. Control ofthe refractive index mismatch, relative layer thicknesses within thefilm, and the amount of induced orientation in the film determines whichwavelengths will be polarized. As with other interference films, thewavelengths of light which are polarized are also dependent on the angleof incidence of the incoming light relative to the surface of thepolarizer.

The birefringent interference polarizer of the present inventionreflects and polarizes a portion of the light incident on its surfacewhile transmitting the remainder of the incident light. Essentially nolight is absorbed by the polarizer. During fabrication, the layerthicknesses of the alternating polymer layers may be controlled so thatthe polarizer transmits only a narrow range of wavelengths whilereflecting and polarizing a broad range. For example, the layers in themultilayer film may be arranged so that their thickness increasesmonotonically through the thickness of the film to produce a layerthickness gradient. This provides broad bandwidth reflective propertiesto the polarizer. Such a polarizer can be used as a band pass filterwhich transmits only a narrow range of wavelengths.

Alternatively, the film can be constructed to polarize and reflect onlya narrow wavelength range while remaining transparent to the remainingportion of incident light. If white light is used as a source, thepolarizer of the present invention will reflect polarized light ofspecific wavelengths in one plane dependent upon the optical thicknessesof the layers, while transmitting the remaining light.

One end use for the polarizer of the present invention is installationon an aircraft or vehicular windshield onto which a "heads-up" displayis projected. The polarizer will reduce the glare component from outsideof the aircraft or vehicle, or from within the aircraft or vehicleitself which is at the same angle as the projected heads-up image. Theuse of the present invention results in increased transmission of otherincident light over that which would be possible using conventionalpolarizers which absorb at least some of the incident light. Another usefor the polarizer of the present invention is as a beam splitter.

In order that the invention may be more readily understood, reference ismade to the following example, which is intended to be illustrative ofthe invention, but is not intended to be limiting in scope.

Example 1

Employing an apparatus as generally described in U.S. Pat. Nos.3,773,882 and 3,759,647, a sheet of a birefringent interferencepolarizing film was prepared. The sheet was approximately 0.003 inchesin thickness and had 385 alternating layers (ABABAB) of polycarbonate(Calibre 300-15, trademark of Dow Chemical Company) and polystyrene(Styron 685D, trademark of Dow Chemical Company).

A 1" by 1" by 0.003" sample of the film was post stretched uniaxially at160° C. (above the glass transition temperature of the two polymers) at650 lb/in² from its original 1" length to a final length of 3" and thenquickly quenched with water to orient the polymers. Final samplethickness averaged 0.0015", and the minimum width of the sample was0.50".

The post-stretch conditions were controlled to provide a final averagelayer thickness of 856.8 angstroms for the polycarbonate layers and873.1 angstroms for the polystyrene layers. These layer thicknesses werecalculated to provide a polarizing film which polarized light in themiddle of the visible spectrum (λ=5500 angstroms) with an f-ratio, asdefined above, of 0.5.

Both polymers had measured refractive indices of about 1.6 in anunoriented condition. However, the polycarbonate was measured to have apositive stress optical coefficient of approximately +5,000 Brewsters,while the polystyrene was measured to have a negative stress opticalcoefficient of approximately -5,000 Brewsters. The degree ofpost-stretching was controlled to provide a refractive index mismatchbetween the two polymers of 0.03 in the plane of orientation.

To determine whether the film acted as a polarizer, two of the 385 layerfilms were laminated prior to uniaxial stretching to orient the polymersin the film. Reflectance at a given wavelength was measured along aplane parallel to the uniaxial stretch and along a plane normal to theplane of uniaxial stretch. As can be seen from the graph of FIG. 1,reflectance differences in the parallel and perpendicular planes over awide range of wavelengths demonstrate that the film was functioning topolarize light.

While certain representative embodiments and details have been shown forpurposes of illustrating the invention, it will be apparent to thoseskilled in the art that various changes in the methods and apparatusdisclosed herein may be made without departing from the scope of theinvention, which is defined in the appended claims.

What is claimed is:
 1. A method of making a birefringent interfacepolarizer comprising the steps of:providing at least first and secondpolymeric materials having respective nonzero stress opticalcoefficients, coextruding said first and second polymeric materials inmultiple alternating layers in which said layers increase in thicknessto produce a player thickness gradient, and stretching said layers suchthat said first and second polymeric materials become oriented andproduce a refractive index mismatch in a first plane which is differentfrom the refractive index mismatch between said first and secondpolymeric materials in a second plane normal to said first plane.
 2. Themethod of claim 1 in which said first and second polymeric materialshave a glass transition temperature and wherein said stretching step iscarried out at a temperature above the glass transition temperature butbelow the melting temperature of said polymeric materials.
 3. The methodof claim 1 in which said first and second polymeric materials havesubstantially equal refractive indices when unoriented.
 4. The method ofclaim 1 in which said refractive index mismatch in one of said planesincreases while said refractive index mismatch in the other planedecreases such that said oriented first and second polymeric materialshave substantially equal refractive indices in one of said planes. 5.The method of claim 1 in which said first and second polymeric materialsare uniaxially oriented.
 6. The method of claim 1 in which said firstpolymeric material has a positive stress optical coefficient and saidsecond polymeric material has a negative stress optical coefficient. 7.The method of claim 1 in which said refractive index mismatch in thefirst plane is at least 0.03.
 8. The method of claim 1 in which theoptical thickness of each layer is from about 0.09 micrometers to about0.70 micrometers.
 9. The method of claim 1 in which said first polymericmaterial is selected from the group consisting of polycarbonates andpolyethylene terephthalates.
 10. The method of claim 1 in which saidsecond polymeric material is selected from the group consisting ofpolystyrene, copolymers of styrene and acrylonitrile, copolymers ofstyrene and methyl methacrylate, and polyethylene naphthalate.
 11. Themethod of claim 1 in which said second polymeric material is asyndiotactic polystyrene.